The primary goal of this proposal is to provide a molecular picture of the basis for the profound effects of cellular crowding and nonideal solution effects in general on biology via the development of new theoretical techniques in conjunction with simulation to provide a molecular picture (theory) of the aspects of the free energy surface that govern protein interfaces and associations in solution. In this project we will produce a molecular picture (theory) of protein interfaces and properties in nonideal solution which are consistent with known and coming thermodynamic and structural data. Test cases including Sm endonuclease and insulin and applications have been chosen to maximize overlap with existing data or collaborations that will yield data of specific relevance to our goal. We will make extensive use of concentration variables in our theoretical and computational studies. Biology rarely uses temperature differences as a mechanism to drive processes; nature more often uses solution composition (concentration of salt, osmolytes, other biomolecules, etc) to affect changes in biological conformation and state of aggregation. We seek to provide a better understanding of these effects.